NREL's New Microscopy Technology Reveals Path to Cheaper Solar Cells

May 17, 2011

The solar cell industry is looking to cut costs in production and manufacturing, and
one avenue is to use lower-cost materials—such as low-cost, solar-grade silicon. However,
silicon of this grade has more impurities than higher-cost silicon, and those impurities
can cause weak junctions and weak regions in the cell (called "shunts") that can lower
reliability of solar modules and lead to their destruction. For example, when a solar
module is partially shaded, the solar cells in the shaded area go into reverse bias
mode, in which current can potentially leak between the positive and negative sides
of the cell. Under high reverse bias mode, a solar cell with shunts will allow an
extremely high current to be driven through the region. This high current heats up
and breaks down the area, causing the module to fail.

To avoid such a scenario, researchers at the National Renewable Energy Laboratory
(NREL) in Golden, Colorado, knew they had to examine this breakdown process at the
microscopic level. But traditional imaging tools, thermography and luminescence imaging,
did not show the shunt areas clearly—all that could be seen was a blurred spot at
the shunt location. Therefore, a research team, led by Manuel Romero decided to create
a new optical tool. Romero's team is part of the Measurements & Characterization group in the National Center for Photovoltaics, which is based at NREL. The team
includes Steve Johnston, Mowafak Al-Jassim, Kirstin Alberi, Charles Teplin, David
Young, and Howard Branz.

Together, they developed a near-field scanning optical microscope (NSOM). This microscope
employs special optics technology to allow observation of the heat in the shunt areas
at extremely high resolution, that is, at a scale of 50 to 100 nanometers. The NSOM
has an optical fiber tip, which researchers use to scan the solar cell at close range.
The tip captures different colors of the light emitted from the shunted area, and
those colors reveal, with unprecedented precision, where the breakdown originates
and how it expands to cover the surrounding area.

The NSOM allowed the team to characterize junction breakdown in solar cells and to
identify the microstructural defects that caused the degradation in open-circuit voltage
and high dark currents in epitaxial silicon solar cells. The team also discovered
the manufacturing processes and issues that caused the defects.

Romero says,"The use of high-resolution microscopies helps our understanding of the
causes behind these shunts, and our findings are shared with our industrial partners
to improve solar cells and modules."

And they aren't stopping there. Romero says he and his team are working hard to develop
more instrumentation to meet the ever-increasing demands imposed by the exploratory
research in photovoltaics—research that is driven by NREL’s and DOE’s missions.

"We continue to develop new techniques in the area of electron microscopy and scanning
probe microscopy to measure all the physical properties of solar cells with high resolution,"
Romero says. It is this kind of scientific innovation that will enable the development
of more economical, market-viable solar cells, and advance the deployment of solar
technology across the nation and the globe.